1. Field of the Invention
The present invention relates to an imaging device and an exposure adjusting method.
2. Description of the Related Art
Conventionally, there is known an imaging device that receives light from an object by spatially splitting the light into a plurality of light beams having different wavelength characteristics and outputs an image including a plurality of areas corresponding to the split light beams (for example, see Japanese Laid-open Patent Publication No. 2013-214950). In the imaging device disclosed in Japanese Laid-open Patent Publication No. 2013-214950, a filter including a plurality of filter areas having different spectral transmission characteristics is arranged at a stage prior to a microlens array in a configuration of a plenoptic camera. A light receiving element array (an imaging element) receives light serially transmitted through the filter and the microlens array, and outputs a light field image, in which partial images (hereinafter referred to as “macro pixels”) corresponding to a plurality of microlenses of the microlens array are arranged.
Each of the macro pixels in the light field image has a plurality of image areas respectively corresponding to the plurality of filter areas of the filter. By obtaining, from the macro pixels, representative values of image areas corresponding to the same filter area and rearranging the representative values in accordance with the positions of the macro pixels, it is possible to generate a plurality of spectral images corresponding to the light beams transmitted through the filter areas. The spectral images may be used to detect a color of an object, for example.
In color detection using the spectral images, if the spectral images contain a pixel in a saturated state, detection accuracy is reduced. Further, a signal-to-noise ratio (S/N) of a pixel with a low luminance value is low; therefore, if each of the pixels of the spectral images do not have adequate luminance, it is difficult to stably detect colors. Therefore, there is a need to appropriately adjust exposure in capturing a light field image so as to enable to generate spectral images appropriate for detecting a color of an object.
As an exposure adjusting method for a multispectral camera, a method disclosed in Japanese Laid-open Patent Publication No. 2007-127657 is known, for example. In the method disclosed in Japanese Laid-open Patent Publication No. 2007-127657, when a reflection spectrum of an object is to be captured while switching among bandpass filters, preliminary exposure is performed to obtain an appropriate exposure time for each of the bandpass filters. Then, the exposure time is changed in synchronization with the switching among the bandpass filters, and imaging is performed with the appropriate exposure time for each of the bandpass filters.
However, in the conventional method of adjusting exposure of a camera, including the method disclosed in Japanese Laid-open Patent Publication No. 2007-127657, an adjustment is made so that exposure is appropriately performed on the entire sensor plane of the imaging element. Therefore, if the conventional exposure adjusting method is applied to an imaging device that generates spectral images from a light field image, exposure is adjusted for an area including an area that is not reflected in the spectral images. Therefore, it is difficult to appropriately adjust an exposure time of the imaging element so as to meet the above described conditions needed for the spectral images.